Carrier Aggregation
The Long Term Evolution (LTE) Release 10 specifications have been standardized, providing support for Component Carrier (CC) bandwidths up to 20 MHz, which is the maximal LTE Rel-8 carrier bandwidth. LTE Rel-10 operation wider than 20 MHz is possible, whereby two or more LTE CCs are used by an LTE Rel-10 terminal. The straightforward way to obtain bandwidths wider than 20 MHz is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs each have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 1.
Release 10 of the LTE standards provides support for up to 5 aggregated CCs, where each CC is limited in the radio-frequency (RF) specifications to have one of six bandwidths, namely 6, 15, 25, 50, 75 or 100 RB, corresponding to 1.4, 3, 5, 10, 15 and 20 MHz respectively. The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink (DL) and uplink (UL) is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different in DL and UL. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal: A terminal may support more downlink CCs than uplink CCs, for example, even though the network offers the same number of uplink and downlink CCs.
CCs are also referred to as cells or serving cells. More specifically, in an LTE network the cells aggregated by a terminal include one carrier denoted the primary component carrier or primary Serving Cell (PCell), and one or more others referred to as secondary component carriers or secondary Serving Cells (SCells). The term “serving cell” comprises both PCell and SCells. All UEs have one PCell; which cell is a UE's PCell is terminal specific. The PCell for a given UE is considered “more important” to the UE than its SCells, since vital control signaling and other important signaling are typically handled via the PCell. Uplink control signaling is always sent on a UE's PCell. The component carrier configured as the PCell is the primary CC whereas all other component carriers are secondary serving cells.
During initial access, an LTE Rel-10 terminal behaves similarly to an LTE Rel-8 terminal. However, upon successful connection to the network a Rel-10 terminal may—depending on its own capabilities and the network—be configured with additional serving cells in the UL and DL. Configuration is done using Radio Resource Control (RRC) signaling. Due to the heavy signaling and rather slow speed of RRC signaling it may typically be the case that a terminal is configured with multiple serving cells, even though not all of them are currently used.
SCell Activation and Deactivation
With the concept of SCells, additional bandwidth resources can be configured and de-configured dynamically, in response to the UE's needs. The configuration and de-configuration of cells is signaled by the eNB and performed with RRC signaling, which is heavy signaling and slow. Since RRC signaling is heavy and slow, a separate concept of activation and deactivation has been introduced for SCells. The eNB has the possibility to deactivate any serving cells that the eNB decides the UE should not use or does not need for the moment. Activation and deactivation of SCells are performed with Medium Access Control (MAC) signaling, which is faster than RRC signaling. Each SCell is configured with a SCellIndex, which is an identifier or so called Cell Index which is unique among all serving cells configured for this UE. The PCell always have Cell Index 0 and SCell can have a integer cell index of 1 to 7.
The Rel-10 Activation/Deactivation MAC Control Element (CE) consists of a single octet containing seven C-fields and one R-field. Each C-field corresponds to a specific SCellIndex and indicates whether the specific SCell is activated or deactivated. The UE will ignore all C-fields associated with Cell indices not being configured. The Activation/Deactivation MAC CE always indicates the activation status of all configured SCells, meaning that if the eNB wants to activate one SCell it has to include all configured SCells, setting the bit corresponding to each one to indicate activated or deactivated, even if the status for the corresponding SCell has not changed.
If a UE's secondary serving cell is activated, the UE has to monitor the Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH) for that serving cell. This implies a wider receiver bandwidth, higher sampling rates, etc., at the UE, resulting in higher power consumption than if that serving cell were deactivated.
Dual Connectivity
In dual connectivity (DC), the UE can be served by two nodes, called the master eNB (MeNB) and the secondary eNB (SeNB). The UE is configured with a PCC from both MeNB and SeNB. The group of cells associated with the MeNB is referred to as the Master Cell Group (MCG), while the group of cells associated with the SeNB is referred to as the Secondary Cell Group (SCG). Thus, the UE is configured with a PCC from each of the MCG and SCG. The PCell from MeNB and SeNB are called PCell and PSCell (Primary SCell) respectively. Sometimes PSCell is referred to as Special SCell. The PCell and PSCell operate the UE typically independently. The UE is also configured with one or more SCCs from each of MeNB and SeNB. The corresponding secondary serving cells served by MeNB and SeNB are called SCell. The UE in DC typically has separate TX/RX for each of the connections with MeNB and SeNB. This allows the MeNB and SeNB to independently configure the UE with one or more procedures e.g. radio link monitoring (RLM), DRX cycle etc on their PCell and PSCell respectively.
Random Access
In LTE, as in any communication system, a mobile terminal may need to contact the network (via the eNodeB) without yet having a dedicated resource in the uplink (from UE to base station). To handle this, a random access procedure is available, whereby a UE that does not have a dedicated uplink resource may transmit a signal to the base station. The first message (MSG1 or preamble) of this procedure is typically transmitted on a special resource reserved for random access, a physical random access channel (PRACH). This channel can, for instance, be limited in time and/or frequency (as in LTE). See FIG. 2. The resources available for PRACH transmissions are identified to mobile terminals as part of the broadcasted system information (or as part of dedicated RRC signaling in some cases, such as in the case of a handover).
In LTE, the random access procedure can be used for a number of different reasons. Among these reasons are:                initial access, for UEs in the LTE_IDLE or LTE_DETACHED states;        an incoming handover;        resynchronization of the uplink;        a scheduling request, for a UE that is not allocated any other resource for contacting the base station; and        positioning.        
The eNodeB can also order the UE, through a PDCCH message, to perform a contention based random access. The contention-based random access procedure used in LTE is illustrated in FIG. 3. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the PRACH to the eNodeB in the Radio Access Network (RAN).
The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2), which includes an initial grant to be used on the uplink shared channel, a temporary Cell Radio Network Temporary Identification (C-RNTI) for the UE, and a time alignment (TA) update. The TA update is based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2 is transmitted in the downlink to the UE and its corresponding PDCCH message cyclic redundancy code (CRC) is scrambled with a Random Access Radio Network Temporary Identifier (RA-RNTI).
After receiving the random access response (MSG2), the UE uses the grant to transmit a message (MSG3) back to the RAN. The MSG3 is used, in part, to trigger the establishment of RRC and in part to uniquely identify the UE on the common channels of the cell. The timing advance command that was provided to the UE in the random access response is applied in the UL transmission in MSG3. The eNodeB can change the resources blocks that are assigned for a MSG3 transmission by sending a UL grant having its CRC scrambled with a Temporary Cell Radio Network Temporary Identifier (TC-RNTI).
The procedure ends with the RAN solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur because each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission at the same time on the Random Access Channel (RACH), there will be contention between these UEs. The RAN resolves this contention using the contention resolution message (MSG4). MSG4, which is sent by the eNodeB for contention resolution, has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned has its PDCCH CRC is scrambled with the TC-RNTI.
The case when contention occurs is illustrated in FIG. 4, where two UEs transmit the same preamble, p5, at the same time. A third UE also transmits at the same RACH, but since it transmits with a different preamble, p1, there is no contention between this UE and the other two UEs.
The UE can also perform non-contention-based random access. A non-contention-based random access or contention-free random access can be initiated by the eNodeB, for example, to get the UE to achieve synchronization in the uplink. The eNodeB initiates a non-contention-based random access either by sending a PDCCH order or indicating it in an RRC message. The latter of these two approaches is used in the case of a handover.
The procedure for the UE to perform contention-free random access is illustrated in FIG. 5. As with the contention-based random access, the MSG2 is transmitted in the downlink to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention-free random access, as for the contention-based random access, the MSG2 contains a timing alignment value. This enables the eNodeB to set the initial/updated timing according to the UEs transmitted preamble. In LTE in Rel-10, the random access procedure is limited to the primary cell only. This means that the UE can only send a preamble on the primary cell. Furthermore, MSG2 and MSG3 are received and transmitted only on the primary cell. However, MSG4 can be transmitted on any downlink cell, in Rel-10.
In LTE Rel-11, the current assumption is that the random access procedure will be supported also on secondary cells, at least for the UEs supporting Rel-11 carrier aggregation. So far, only network-initiated random access on SCells is assumed.
Random Access Response Window
After a UE has sent a random access preamble, it listens for a random access response from the network for a certain time, which is given by a value for a parameter referred to as the random access response window. After a time equal to the random access response window has passed, the UE considers the preamble transmission unsuccessful and resends the preamble. Each time the UE resends the preamble, the UE increases the output power used to transmit the preamble to increase the chance of a successful preamble transmission. The UE transmits a maximum number of preambles given by a value referred to as the preamble transmission maximum.